Image forming apparatus

ABSTRACT

An image forming apparatus capable of decreasing the image unevenness of a superimposed image at the time of forming starting (head) lines of the images in respective colors with different beams in order to decrease a shift of a superimposed image. Let a position, where an image in a first color is formed with a beam A as a leading beam, be (a). Let a starting position of formation of an image in a second color be (b), and one dot (line) of shift arises in the superimposed image. In order to decrease the shift of the superimposed image, image formation is performed with selecting a beam B as a beam which writes a head of the image (c). When image formation is performed for color shift reduction with changing a leading beam for each color image, tonality correction is performed to each beam for subsequent images so as to decrease image unevenness due to the difference between shapes and exposure power of respective beams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/870,958, filed Jun. 21, 2004, which is based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2003-196266, filed on Jul. 14, 2003, the entire contents of each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, which usesa plurality of beams, such as a copier, a facsimile machine, and aprinter, and in particular, to a method of improving the image qualityof an image formed by the image forming apparatus.

2. Description of the Background Art

Conventionally, many image forming apparatuses of this type which scanwith a plurality of beams concurrently and select a leading beam fromamong the plurality of beams such that color shifts may be prevented aredisclosed in, for example, Japanese Patent Laid-Open No. 10-104537 andJapanese Patent Laid-Open No. 2002-072607. In this prior art, when imageformation is performed by using a plurality of beams, writing is startedfrom a headline of the image in each color with a different beam so asto decrease shifts between superimposed images. Nevertheless, when thewriting of the beginning line of an image, i.e., a top line is startedwith different beams, there is a possibility that image unevennessarises in a superimposed image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image formingapparatus capable of decreasing the image unevenness of superimposedimages when forming the beginning (head) lines of images in respectivecolors with beams which are different in order to decrease a shift of asuperimposed image.

In accordance with the present invention, there is provided an imageforming apparatus which generates a toner image from a latent image,formed on an image carrier by scan type writing means, by developingmeans, repeats multiple times steps of transferring the toner image onan intermediate transfer member for respective colors, and forms a colorimage with superimposing the toner images by turns for respectivecolors. The writing means to form the latent image is constituted suchthat a plurality of beams which is adjacent in a subscanning directionmay perform concurrent scans, and can select from the plurality of beamsa beam which becomes a head at the time of formation of an image in eachcolor to perform tonality correction of each beam according to theselected beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

FIG. 1 is a diagram showing an example of the basic construction of animage forming apparatus according to the present invention;

FIGS. 2A to 2C are views showing schematically lines (solid) formed bydots of beams A and B;

FIG. 3 is a graph showing an example of a γ curve;

FIGS. 4A to 4D are views showing an example of the timing (dotpositions) of scanning beams to a belt mark signal;

FIG. 5 is a block diagram showing an example of the construction of beamselection control means;

FIG. 6 is a view for describing the operation of the beam selectioncontrol means;

FIG. 7 is a view for explaining the operation of beam selection;

FIG. 8 is a block diagram showing another example of the construction ofthe beam selection control means;

FIG. 9 is a timing chart for describing the operation of the beamselection control means shown in FIG. 8; and

FIG. 10 is a diagram showing the construction of an image formingapparatus having two stations.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereafter, the present invention will be described specifically withreference to accompanying drawings.

FIG. 1 shows an example of the basic construction of an image formingapparatus according to the present invention. The image formingapparatus forms a color image by generating a toner image by developingmeans from a latent image formed on an image carrier by scan typewriting means, repeating multiple times a step of transferring the tonerimage on an intermediate transfer member for every color, andsuperimposing the toner image by turns for every color.

As shown in FIG. 1, charging means 2, writing means 3, developing means4, transfer means 5, and cleaning means 6 are located around an imagecarrier 1 such as a photoconductive drum. In addition, an intermediatetransfer member 7 such as an endless belt is located above the imagecarrier 1. Furthermore, the intermediate transfer member 7 is providedwith a mark indicating a reference position.

In operation, a surface of the image carrier 1 which rotates in thedirection shown by an arrow is charged by the charging means 2. When themark on the intermediate transfer member 7 is detected, the writingmeans 3 starts forming a latent image on the image carrier 1. The latentimage on the image carrier is transformed into a visualized image by thedeveloping means 4 as a toner image, and the toner image is transferredto the intermediate transfer member 7 by the transfer means 5. Theresidual toner on the image carrier 1 after transfer is removed by thecleaning means 6.

When a color image is formed, the above-mentioned developing steps arerepeated by the number of times of necessary colors by switching thedeveloping means 4 with switching means not shown and the image in eachcolor is superimposed on the intermediate transfer member 7.

The image superimposed on the intermediate transfer member 7 istransferred to a recording medium such as transfer sheet by anothertransfer means not shown, and is fixed thereto by fixing means notshown.

The image formation on each color is started on the basis of the mark onthe intermediate transfer member 7. When the writing means 3 is a laserscanning optical system, the mark detection in the intermediate transfermember 7 and a synchronizing signal as a writing reference of thewriting means 3 are asynchronous. Hence, even if the image formation isstarted on the basis of the mark of the intermediate transfer member 7,a shift occurs between the superimposed images in respective colors.

An image forming apparatus provided with writing means, which uses aplurality of light sources and performs simultaneous scans with aplurality of beams adjoining in a subscanning direction, decreases theshift of the superimposed images by selecting abeam which forms an imagehead line.

Hereinafter, an example of an apparatus will be described with referenceto the drawings. The apparatus performs simultaneous scans of two linesof beams A and B in one scan with the writing means to form an image andsuperimposes images in respective colors on the basis of a referencesignal such as a belt mark.

FIGS. 2A to 2C schematically show lines (solid) formed by dots ofrespective beams. Arrangement is made such that the beam A may precedethe beam B in a subscanning direction when an image carrier is scanned.In this diagram, a dot line pair shown as the beam A and beam B isformed by a first scan, and dot line pairs of the beams A and Bfollowing the above-mentioned dot line pair therebelow are formed inorder by second and later scans.

It is ideal to start to write all the images in respective colors fromthe same beam, for example, the beam A. However, there is a case thatheadlines of images may be written with different beams for reduction ofthe shift of a superimposed image.

It is assumed that a position where an image in a first color (firstrotation) is formed with the beam A (black dot) as a leading beam is,for example, a position shown in FIG. 2A when performing the imageformation with a first synchronizing signal to a mark signal. At thistime, if a position where the formation of an image in a second color(second rotation) starts is one shown in FIG. 2B, a shift by one dot(line) occurs in a superimposed image. As shown in FIG. 2C, when imageformation is performed by selecting the beam B as the beam as the headof an image, the shift of a superimposed image may be decreased. Thebeam selection is performed by sending empty data, i.e., dummy data tothe beam A, and supplying to the beam B the data for the headline of theimage.

That is, an even line and an odd line in the images in the first andsecond colors, are formed with different beams. Although there is noproblem if dot shapes and exposure power of the beams A and B arecompletely the same, they are different actually.

It is noted that unevenness appears in a superimposed image of theimages in the first and second colors. Then, when image formation isperformed by changing a leading beam for each color image for colorshift reduction, tonality correction is performed to each beam forsubsequent images so as to decrease image unevenness due to thedifference between shapes and power of respective beams.

The tonality correction is performed by modifying a γ curve or theemission power of a light source (LD) which generates a beam. Forexample, a modification may be made so that a portion of γ curve(represented as solid line in FIG. 3) at low tonality region can becompensated, in the case where beams are superimposed which have thinnershapes at shorter lighting times in pulse width modulation, where thereis no change in the dot shape in full lighting (pulse width: 1.0) forone dot of beam. Alternatively, the difference in dot shapes may bereduced by presetting the emission power of beams higher to some extent.

The modification table of the γ curve and the corrective values for theemission power change, described above, are obtained beforehand byoutputting superimposed images for beams. In addition, a change of imageprocessing is also made depending on the case. When the leading beam ofan image to be formed is the same as that of a preceding color image,the tonality correction mentioned above is not performed. Third andlater colors are performed similarly.

Also when the number of beams scanning concurrently is three or more,correction is made for each of beams that form dots overlapping in thesubsequent image formation, thereby reducing image unevenness in thecase where the order of formation beams depends on the color.

A specific method of selecting a leading beam will be described.

Timing of scanning beams with respect to a belt mark signal, which isthe starting reference of image formation in each color, represented asdot positions, is illustrated in FIGS. 4A to 4D. Hereinafter, thedescription is made while the generating timing of a synchronizingsignal of the writing means is represented by the centers of dots.

Here, FIG. 4A shows the belt mark signal. In addition, time T in FIG. 4Bis a line period. In addition, reference character A1 denotes a dotposition formed by the beam A in a first scan, B1 denotes a dot positionformed by the beam B in a fist scan, A2 denotes a dot position formed bythe beam A in a second scan, and B2 denotes a dot position formed by thebeam B in a second scan.

Suppose that the formation of an image in a first color is started inthe timing t1 in FIG. 4B with respect to the belt mark. In the imageformation in a second or later color, when t1−t2>3T/4 as shown in FIG.4C, the image formation is started with one-scan delayed scanning. Thatis, a first line of image data is written from a position A2 shown inFIG. 4C. This allows the reduction of color shift between superimposedimages. In this case, tonality correction is not performed since thebeam at the image head is the same beam A as that in the first color.

In addition, when T/4<t1−t3<3T/4 as shown in FIG. 4D, image data iswritten from a position B1. Here, a headline of the image is writtenwith the beam B different from the beam A with which the headline of theimage in the first color was formed. Therefore, the tonality correctionis performed. Note that, in the case of t1−t3<T/4, image formation isstarted from the position Al which is the same as that in the firstcolor.

Here, if the image in the first color is formed in the timing in FIG.4C, start timing in the second and later color as shown in FIG. 4B makeit impossible to superimpose head lines. Then, image formation startswith a synchronized detection signal after a preset reference value, orT/2 in this case, has elapsed from the belt mark detection.

FIG. 5 shows the construction of beam selection control means whichperforms the control mentioned above. It comprises: mark detection means21 for detecting a mark of the intermediate transfer member 7; firstmeasuring means 22 for measuring elapsed time after mark detectionwhenever detecting the mark used as the reference of image formationstart; first determining means 24 comparing a first reference value, setbeforehand, with a measured value of the first measuring means 22 andjudging their magnitude; first memory means 25 storing time (t1) fromdetecting the mark to a synchronizing signal when or after a measuredvalue of the first measuring means 22 reached the first reference value;calculation means 26 obtaining a difference between measured time (t2,t3) in the first measuring means 22 from detecting the mark of the imagestarting reference in the second and later color to a synchronizingsignal generated by the writing means, and the measurement result of thefirst memory means 25; and calculation means 26 outputting its absolutevalue and a positive or negative; second judging means 27 comparing theresult of the calculation means 26 with a second reference value setbeforehand and judging their magnitude; third judging means 28 comparingthe result of the calculation means 26 with a third reference value setbeforehand and judging their magnitude; fourth judging means 29comparing the result of the calculation means 26 with a fourth referencevalue set beforehand and judging their magnitude; and beam selectionunit 30 commanding beam selection to the writing controller from theresult of the positive or negative of the difference, obtained by thecalculation means 26, and the result of the second judging means 27,third judgment means 28, and fourth judging means 29.

Here, the first reference values is T/2 that is a half of a period T ofthe synchronizing signal of the writing means 3, the second referencevalue of the second judging means 27 is 5T/4, the third reference valueof the third judging means 28 is 3T/4, and the fourth reference value ofthe fourth judging means 29 is T/4.

The control means in FIG. 5 will be described by using FIG. 6.

By the first determining means 24, the writing of a first color image isstarted with a synchronizing signal at the time when T/2 elapses afterthe detection of the mark signal shown by (a) in FIG. 6. Hence, thestart timing of the image in the first color may occur between (b) and(c). Here, (b) is the case that a synchronizing signal occursimmediately after T/2 elapses, and (c) is the case that a synchronizingsignal occurs just before T/2 elapses.

Since the writing means forms two lines in one scan, dot positionsgenerated in respective timing are made as shown by (f) and (i) in FIG.6. An arrow denotes a subscanning direction.

The first memory means 25 measures and stores a time period from thebelt mark to the writing start with the synchronizing signal after thefirst reference value (T/2) elapses (for example, t10min or t10max). Thestart timing of images in second and later colors is changed in a periodbetween (d) and (e) in FIG. 6 at the maximum similarly to the above.

The first measuring means 22 measures a time period from the detectionof the mark to the generation of the synchronizing signal of the writingmeans 3 (for example, t20min or t20max). In addition, the calculationmeans 26 subtracts a time period until the generation of thesynchronizing signal of the writing means 3 for second and later colors(for example, t20min or t20max) from a time period until the writingstart of the image in the first color which the first memory means 25measures and stores (for example, t10min or t10max), and obtains anabsolute value and determines the sign such as positive or negative.

The second judging means 27 judges the magnitude between the result ofthe calculation means 26 and 5T/4 which is the second reference value,and outputs the result to the beam selection unit 30. The third judgingmeans 28 judges the magnitude between the result of the calculationmeans 26 and 3T/4 which is the third reference value, and outputs theresult to the beam selection unit 30. The fourth judging means 29 judgesthe magnitude between the result of the calculation means 26 and T/4which is the fourth reference value, and outputs the result to the beamselection unit 30.

In addition, the calculation means 26 outputs the positive or negativesign of the difference to the beam selection unit 30. According to theresult of each judging means and calculation means, the beam selectionunit 30 selects a leading beam of images in the second and later colorsas shown in FIG. 7, and controls the outputting order of image data inconnection with it. Here, suppose that the beam A is used for theformation of a headline in the first color.

The operation of beam selection will be described with reference to FIG.7. That is, if the result of the second judging means 27 is “larger”,the beam selection unit 30 outputs head line data to a subsequent beam(beam B) that is in a scan operation delayed by one scan from thesynchronizing signal immediately after the belt mark detection to startimage formation. If the result of the second judging means 27 is“smaller” and the result of the third judging means 28 is “larger”, thebeam selection unit 30 starts image formation in a scan operationdelayed by one scan from the synchronizing signal immediately after thebelt mark detection. A leading beam at this time is the same as that inthe first color.

If the result of the second judging means 27 and the result of the thirdjudging means 28 are “smaller” and the result of the fourth judgingmeans 29 is “larger”, the beam selection unit 30 selects a beamaccording to the positive or negative of the difference obtained by thecalculation means 26. If the difference is “positive”, the beamselection unit 30 outputs head line data to a subsequent beam (beam B),and starts image formation. If the difference is “negative”, the beamselection unit 30 outputs second line data to a leading beam (beam A),and starts the image formation.

When all the results of the second judging means 27, third judging means28, and fourth judging means 29 are “smaller”, the beam selection unit30 starts the image formation from the same leading beam (beam A) asthat in the first color.

A specific example is shown in FIG. 6. Suppose that the start of imagesin second and later colors is (d) or (e) when the formation of an imagein the first color is started in the timing of (b) in FIG. 6, In thecase of (d), since time difference Δt holds T/4<Δt<3T/4, the results ofthe second judging means and third judging means are “smaller”, theresult of the fourth judging means is “large”, and Δt is “positive”.Hence, the beam selection unit 30 writes headline data from a positionof a consecutive beam B (dot G2).

In the case of (e), the result of each judging means is the same as thecase of (d), but since Δt is “negative”, image formation is performedfrom the second line data of an image in a position of the leading beamA (dot H1).

Owing to this, the head line data of the image data in the second andlater colors can be written in a position G2 to the head line position(dot F1) in the first color, or the second line data in the second colorcan be written in a position H1 to a dot position F2 of the second linein the first color. Hence, it is possible to decrease a positionalerror.

In addition, when the first color starts in the timing (dot I1) of (c),the result of the second judging means 27 is “larger” when the start ofimages in second and later colors is in (d). Hence, the beam selectionunit 30 starts writing with the consecutive beam B from a secondsynchronizing signal (delayed by one scan) after the mark detection.Also even when the number of scanning beams is three or more, similarcontrol may be performed for reducing the position shift, by providing aplurality of reference values and judging means as mentioned above andsetting conditions adequately.

In the previous description, although the formation of an image in thefirst color is started with a first synchronizing signal after the firstreference value (here, T/2) elapses, there may be another examples. Asetting value n is provided such that writing can be started withdelaying a synchronizing signal of the writing means by n periods. FIG.8 shows other construction of beam selection control means of thepresent invention. In this beam selection control means, counting means18 is further added to the construction in FIG. 5.

The counting means 18 is provided which counts the number ofsynchronizing signals of the writing means after the first referencevalue (period) elapses after the detection of the mark used as a writingstarting reference during the formation of an image in the first color,and counts the number of synchronizing signals after the detection ofthe mark during the formation of images in second and later colors. Thecounting means is set at n, and the counting means instructs the writingcontroller to start image formation when the counted value reaches n.For example, when n=3, the formation of an image in the first color isstarted from pe when a synchronizing signal for the first color is thetiming shown by (b) in FIG. 9. Moreover, in regard to the formation ofimages after second and later colors, calculation is performed withmaking pg3 a reference synchronizing signal when it is the timing of(c), or making ph3 a reference synchronizing signal when it is thetiming of (d). Furthermore, image formation is started from the resultof the calculation on the basis of FIG. 7.

Thus, the writing start position can be changed thereby avoiding theimage formation on a connecting portion of an intermediate transfermember and use in the same portion.

FIG. 10 shows the construction of the image forming apparatus with twostations according to the present invention. This image formingapparatus has two image forming means (first station for (C, M) andsecond station for (Y, K)) under the intermediate transfer member 7.Each image forming means comprises: one image carrier 1; writing beams3; developing means 4 comprising at least two developing units each fordeveloping an electrostatic latent image formed on this image carrier byeach beam 3; and switching means which alternatively selects and drivesa developing unit of the developing means 4. This image formingapparatus generates a multiple-color image by superimposing imagesformed by a plurality of above-mentioned image forming means, on anintermediate transfer member. Therefore, according to the presentinvention, it becomes possible to easily superimpose toner images formedon the intermediate transfer member in respective colors in adequateaccuracy, and hence, it is possible to achieve a high-definition fullcolor image forming apparatus that is free of image unevenness.

As described above, the present invention has the following advantageousfeatures:

(1) Different beams that form superimposed lines can also provide asuperimposed image with reduced image unevenness.

(2) It is possible to prevent adverse effects caused by needlesstonality correction processing.

(3) A simple construction can achieve the controlled reduction ofposition (color) shift of a superimposed image, and suppress imageunevenness.

(4) Even when the writing means with a two-line simultaneous scan isused, it is possible to decrease a position (color) shift of asuperimposed image easily. In addition, it is possible to reduce thevariation in position shifts of superimposed images.

(5) It is possible to reduce the image unevenness of a superimposedimage easily.

(6) It is possible to reduce a position (color) shift of a superimposedimage easily, and to suppress image unevenness.

(7) Since change an image formation position on an intermediate transfermember can be changed, it is possible to prevent the degradation of theintermediate transfer member.

(8) Since the image formation position on the intermediate transfermember can be changed depending on the operating conditions, it ispossible to prevent the degradation of the intermediate transfer member.

(9) Also in an image forming apparatus which is small, high-speed, andlow cost, it is possible to form a high-definition image without imageunevenness where a position shift (color shift) of superimposed imagesis reduced.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

1. An image forming method for generating a toner image from a latentimage formed on an image carrier by a scan type writer and developer,the method comprising: concurrently performing scans to form a latentimage with a plurality of beams, the beams being adjacent in asubscanning direction; selecting from the plurality of beams a beamwhich forms a headline at a time of formation of an image in each colorto perform tonality correction on each beam to change beam emissioncharacteristics of each beam to beam emission characteristics of theselected beam; developing the latent image to form a toner image;transferring the toner image onto an intermediate transfer member oncefor each respective color; and forming a color image by superimposingthe toner images one at a time for each respective color.
 2. The imageforming method as claimed in claim 1, wherein the tonality levelcorrection is not performed when a beam which forms a headline of aselected image is the same as a beam for an image formed previously. 3.The image forming method as claimed in claim 1, further comprising:detecting a mark used as a starting reference of an image in each colorwhich is provided on an intermediate transfer member; measuring a firsttime as an elapsed time after each mark detection, said measuringrestarting each time the mark is detected; calculating a differencebetween a first reference value, set beforehand, and the first time;measuring and storing a second time from detecting the mark to asynchronizing signal when or after the first time reaches the firstreference value; calculating a third time as an absolute value of adifference between a time from detecting the mark used as the startingreference to a synchronizing signal generated by a writing mechanism ina second or later color and the second time; calculating a differencebetween the third time and a second reference value set beforehand;calculating a difference between the third time and a third referencevalue set beforehand; calculating a difference between the third timeand a fourth reference value set beforehand; synchronizing the start ofwriting of an image in a first color with a synchronizing signal of thewriting mechanism when the first time is larger than the first referencevalue; selecting a leading beam of an image in a second or later coloraccording to the difference between the third time and the secondreference value, the difference between the third time and the thirdreference value, the difference between the third time and the fourthreference value, and the difference between the time from detecting themark used as the starting reference in a second or later color to asynchronizing signal generated by the writing mechanism and the secondtime.
 4. The image forming method as claimed in claim 3, furthercomprising: using two beams as the plurality of beams for a scan;generating a writing synchronizing signal with a scanning beam with aperiod of T, wherein the first reference value is T/2, the secondreference value is 5T/4, the third reference value is 3T/4, and thefourth reference value is T/4.
 5. The image forming method as claimed inclaim 3, further comprising: when starting formation of an image in asecond or later color, controlling start data so as to output empty datato a leading beam A and output headline data of an image to a subsequentbeam B with delaying a page headline by one scan when the third time isgreater than the second reference value, to output line data withdelaying a page headline by one scan when the third time is greater thanthe third reference value and the third time is less than the secondreference value, to output empty data to a leading beam A and outputheadline data of an image to a subsequent beam B when a differencebetween the time from detecting the mark used as the starting referencein a second or later color to the synchronizing signal generated by thewriting mechanism and the second time is positive or to output imagedata in a second line to a leading beam A when the difference betweenthe time from detecting the mark used as the starting reference in asecond or later color to the synchronizing signal generated by thewriting mechanism and the second time is negative, when the third timeis greater than the fourth reference value and the third time is lessthan the third reference value, and to output image data in the sameline as that in the first color when the third time is less than thefourth reference value.
 6. The image forming method as claimed in claim3, further comprising: counting a number n as the number ofsynchronizing signals detected from a detection of the mark until afirst reference value after detection of the mark; starting writing animage in a first color when the count reaches n; and controlling writingstart timing in a second or later color so as to start writing of animage in synchronization with an n-th synchronizing detection signal ofthe writing mechanism after mark detection when the third time is lessthan the third reference value, or to start writing of an image insynchronization with an (n+1)-th synchronizing signal of the writingmechanism after mark detection when the third time is greater than thethird reference value.
 7. The image forming method as claimed in claim6, further comprising: setting n based on at least one of environmentaltemperature, print amount, and operating time.
 8. The image formingmethod as claimed in claim 1, further comprising: performing thetonality correction by changing a γ correction table of each beam. 9.The image forming method as claimed in claim 1, further comprising:performing the tonality correction by changing the emission power ofeach beam.
 10. The image forming method as claimed in claim 1, whereinthe scan type writer and developer includes an intermediate transfermember and a plurality of image forming units facing a moving face ofthe intermediate transfer member, wherein each image forming unitcomprises one image carrier, one writing mechanism, at least twodevelopers which each develop an electrostatic latent image formed onthe image carrier by a plurality of beams emitted from the writingmechanism, and a switch which alternatively selects and drives thedeveloper, the method further comprising: forming a color image bytransferring a toner image formed by the image forming unit located atan upstream position to the intermediate transfer member; andsuperimposing toner images formed by the image forming units located atdownstream positions in order on the intermediate transfer member. 11.An image writing apparatus comprising: a writing mechanism including aplurality of beams, the beams being adjacent in a subscanning directionand configured to perform concurrent scans, the writing mechanismconfigured to select from the plurality of beams a beam which forms aheadline at a time of formation of an image in each color to performtonality correction on each beam to change beam emission characteristicsof each beam to beam emission characteristics of the selected beam. 12.The image writing apparatus as claimed in claim 11, wherein the writingmechanism does not perform a tonality level correction when a beam whichforms a headline of a selected image is the same as a beam for an imageformed previously.
 13. The image writing apparatus as claimed in claim11, further comprising: an intermediate transfer member configured toreceive multiple transfers of a toner image for respective colors, suchthat a color image is formed by superimposing the toner images by turnsfor respective colors; mark detection means detecting a mark used as astarting reference of an image in each color which is provided on theintermediate transfer member; first measuring means measuring as a firsttime an elapsed time since mark detection, said first time reset at eachmark detection; first judging means calculating a difference between afirst reference value, set beforehand, with the first time; first memorymeans measuring and storing as a second time a time from detecting themark to a synchronizing signal when or after the first time reaches thefirst reference value; calculation means obtaining a third time as anabsolute value of a difference between a time from detecting the mark ofthe image starting reference to a synchronizing signal generated by thewriting mechanism in a second or later color and the second time; secondjudging means calculating a difference between a second reference valueset beforehand and the third time; third judging means calculating adifference between a third reference value set beforehand and the thirdtime; and fourth judging means calculating a difference between a fourthreference value set beforehand and the third time, wherein writing of animage in a first color is started with synchronizing with asynchronizing signal of the writing mechanism when the first judgingmeans judges that the first time is greater than the first referencevalue, and wherein a leading beam of an image in a second or later coloris selected according to a result of the second judging means, a resultof the third judging means, and a result of the fourth judging means,and a result of a positive or negative result of a difference obtainedby the calculation means.
 14. The image writing apparatus as claimed inclaim 13, wherein a number of a plurality of beams for a scan is two, aperiod of a writing synchronizing signal which a scanning beam generatesis T, the first reference value is T/2, the second reference value is5T/4, the third reference value is 3T/4, and the fourth reference valueis T/4.
 15. The image writing apparatus as claimed in claim 13, whereinin regard to start of formation of an image in a second or later color,start data is controlled so as to output empty data to a leading beam Aand output headline data of an image to a subsequent beam B withdelaying a page headline by one scan when the third time is greater thanthe second reference value, to output line data with delaying a pageheadline by one scan when the third time is greater than the thirdreference value and the third time is less than the second referencevalue, to output empty data to a leading beam A and output headline dataof an image to a subsequent beam B when a difference between the timefrom detecting the mark used as the starting reference in a second orlater color to the synchronizing signal generated by the writingmechanism and the second time is positive or to output image data in asecond line to a leading beam A when the difference between the timefrom detecting the mark used as the starting reference in a second orlater color to the synchronizing signal generated by the writingmechanism and the second time is negative, when the third time isgreater than the fourth reference value and the third time is less thanthe third reference value, and to output image data in the same line asthat in the first color when the third time is less than the fourthreference value.
 16. The image writing apparatus as claimed in claim 13,further comprising: counting means for counting the number ofsynchronizing signals after the first time reaches a first referencevalue at the time of writing of an image in a first color, wherein thenumber of synchronizing signals after mark detection at the time ofwriting in a second or later color is counted, writing of an image in afirst color is started when a count of the counting means reaches n (nis a positive integer), and writing start timing in a second or latercolor is controlled so as to start writing of an image withsynchronizing with an n-th synchronizing detection signal of the writingmechanism after mark detection when the third time is less than thethird reference value, or to start writing of an image withsynchronizing with an (n+1)-th synchronizing signal of the writingmechanism after mark detection when the third time is greater than thethird reference value.
 17. The image writing apparatus as claimed inclaim 16, wherein n is based on at least one of environmentaltemperature, print amount, and operating time.
 18. The image writingapparatus as claimed in claim 11, wherein the tonality correction isperformed by changing a γ correction table of each beam.
 19. The imagewriting apparatus as claimed in claim 11, wherein the tonalitycorrection is performed by changing the emission power of each beam. 20.The image writing apparatus as claimed in claim 11, further comprising:an intermediate transfer member; and a plurality of image writing unitswhich are arranged facing a moving face of the intermediate transfermember, each image writing unit including, one image earner, one writingmechanism, at least two developers which each develop an electrostaticlatent image formed in the image carrier by a plurality of beams emittedfrom the writing mechanism, and a switch which alternatively selects anddrives the developing means, wherein the image writing apparatus forms acolor image by transferring a toner image formed by an image writingunit located in an upstream position to the intermediate transfermember, and toner images formed by image writing units located indownstream positions are superimposed in order on the intermediatetransfer member.
 21. An image writing method for forming a latent imageon an image carrier by a scan type writer, the method comprising:concurrently performing scans to form a latent image with a plurality ofbeams, the beams being adjacent in a subscanning direction; andselecting from the plurality of beams a beam which forms a headline at atime of formation of an image in each color to perform tonalitycorrection on each beam to change beam emission characteristics of eachbeam to beam emission characteristics of the selected beam.
 22. Theimage writing method as claimed in claim 21, wherein the tonality levelcorrection is not performed when a beam which forms a headline of aselected image is the same as a beam for an image formed previously. 23.The image writing method as claimed in claim 21, further comprising:developing the latent image to form a toner image; transferring thetoner image onto an intermediate transfer member once for eachrespective color; detecting a mark used as a starting reference of animage in each color which is provided on the intermediate transfermember; measuring a first time as an elapsed time after each markdetection, said measuring restarting each time the mark is detected;calculating a difference between a first reference value, setbeforehand, and the first time; measuring and storing a second time fromdetecting the mark to a synchronizing signal when or after the firsttime reaches the first reference value; calculating a third time as anabsolute value of a difference between a time from detecting the markused as the starting reference to a synchronizing signal generated by awriting mechanism in a second or later color and the second time;calculating a difference between the third time and a second referencevalue set beforehand; calculating a difference between the third timeand a third reference value set beforehand; calculating a differencebetween the third time and a fourth reference value set beforehand;synchronizing the start of writing of an image in a first color with asynchronizing signal of the writing mechanism when the first time islarger than the first reference value; selecting a leading beam of animage in a second or later color according to the difference between thethird time and the second reference value, the difference between thethird time and the third reference value, the difference between thethird time and the fourth reference value, and the difference betweenthe time from detecting the mark used as the starting reference in asecond or later color to the synchronizing signal generated by thewriting mechanism and the second time; and forming a color image bysuperimposing the toner images one at a time for each respective color.24. The image writing method as claimed in claim 23, further comprising:using two beams as the plurality of beams for a scan; generating awriting synchronizing signal with a scanning beam with a period of T,wherein the first reference value is T/2, the second reference value is5T/4, the third reference value is 3T/4, and the fourth reference valueis T/4.
 25. The image writing method as claimed in claim 23, furthercomprising: when starting formation of an image in a second or latercolor, controlling start data so as to output empty data to a leadingbeam A and output headline data of an image to a subsequent beam B withdelaying a page headline by one scan when the third time is greater thanthe second reference value, to output line data with delaying a pageheadline by one scan when the third time is greater than the thirdreference value and the third time is less than the second referencevalue, to output empty data to a leading beam A and output headline dataof an image to a subsequent beam B when a difference between the timefrom detecting the mark used as the starting reference in a second orlater color to the synchronizing signal generated by the writingmechanism and the second time is positive or to output image data in asecond line to a leading beam A when the difference between the timefrom detecting the mark used as the starting reference in a second orlater color to the synchronizing signal generated by the writingmechanism and the second time is negative, when the third time isgreater than the fourth reference value and the third time is less thanthe third reference value, and to output image data in the same line asthat in the first color when the third time is less than the fourthreference value.
 26. The image writing method as claimed in claim 23,further comprising: counting a number n as the number of synchronizingsignals detected from a detection of the mark until a first referencevalue after detection of the mark; starting writing an image in a firstcolor when the count reaches n; and controlling writing start timing ina second or later color so as to start writing of an image insynchronization with an n-th synchronizing detection signal of thewriting mechanism after mark detection when the third time is less thanthe third reference value, or to start writing of an image insynchronization with an (n+1)-th synchronizing signal of the writingmechanism after mark detection when the third time is greater than thethird reference value.
 27. The image writing method as claimed in claim26, further comprising: setting n based on at least one of environmentaltemperature, print amount, and operating time.
 28. The image writingmethod as claimed in claim 21, further comprising: performing thetonality correction by changing a γ correction table of each beam. 29.The image writing method as claimed in claim 21, further comprising:performing the tonality correction by changing the emission power ofeach beam.
 30. The image writing method as claimed in claim 21, whereinthe scan type writer includes an intermediate transfer member and aplurality of image writing units facing a moving face of theintermediate transfer member, wherein each image writing unit comprisesone image carrier, one writing mechanism, at least two developers whicheach develop an electrostatic latent image formed on the image carrierby a plurality of beams emitted from the writing mechanism, and a switchwhich alternatively selects and drives the developer, the method furthercomprising: forming a color image by transferring a toner image formedby the image writing unit located at an upstream position to theintermediate transfer member; and superimposing toner images formed bythe image writing units located at downstream positions in order on theintermediate transfer member.